Download Effects and Sites of Action of Different Agonists Drugs (Stimulant or

313 PHL
Lab # 1
Effects and Sites of Action of Different Agonists
Drugs (Stimulant or Relaxant)on The
Isolated Rabbit Intestine
Purpose of Pharmacological Experiments :
The pharmacological experiments largely divided
in to :
Qualitative :
- To determine ( analyze) the activity of drug (i.e. analgesic,
hypotensive and hypoglycemic).
-To determine mechanism of drugs action.
Quantitative:
To assay (measure) the potency of drugs (e.g. toxic level, therapeutic
level, LD50).
Types of pharmacological experiments :
A- In vitro experiments
- Study the action of drugs on isolated preparation:
1) Tissue e.g. (intestine, ileum) .
2) Organ e.g. (heart) .
• Advantages:
- Easier to study MOA (qualitative) .
- Easier to study its potency (quantitative) e.g. DRC on rectus
abdominis.
• Disadvantages :
- There is no interference by compensatory mechanisms of body
that may affect the direct action of drug  not easy to study the
chemical and biological effect of the drug.
B- In vivo experiments
- Study the action of drugs on the whole body (intact) .
• Advantages:
- Observe the direct drug action(actual effect) .
- Easier to study the chemical and biological effect .
• Disadvantages :
- Difficulty to determine the exact mechanism of action .
In vitro experiments
1) Experimental conditions:
An isolated organ or muscle should be bathed in a suitable media that
is resemble to the normal condition in the body regarding
(pH,osmolarity,temperature) .
There fore, tissue is bathed in solution containing:
1- Electrolytes .
2- Nutrients .
3- Adjusted Physiological pH .
4- Constant temperature .
5- Aeration .
2) Physiological Salt Solution :
• According to different tissue to be used, there is a different
physiological solution prepared.
• But all or most of them contain the following ingredients at different
concentrations:
1- Nacl → to adjust the isotonicity.
2- Glucose → nutrient .
3- Ca+2, K+, sometimes Mg+2 as CL salt → to maintain normal
muscle contraction.
3- Sodium bicarbonate → to adjust pH similar to that of blood .
4- NaH2PO4 (sodium di hydrogen phosphate) buffer→ to
compensate for any change in pH .
The two most commonly used physiological solutions are Tyrode’s
and Krebs .
3) Aeration:
Physiological salt solutions should be aerated by a suitable gas :
1- Pure oxygen → for heart tissues.
2- Normal air → for intestine.
3- Carbogen (95% O2, 5% CO2) → used for uterus .
3)Temperature:
• Mammalian tissue must be bathed in a warmed solution adjusted
at 37ºC e.g. Isolated rabbit intestine .
• The temperature should be decreased in some experiments to
reduce the myogenic contraction, the warmed solution adjusted at
32ºC e.g. Guinea-pig ileum.
• Amphibian tissues survive at room temperature (25ºC) e.g. frog
rectus abdominis .
Dose cycle
Normal time (NT): the time between different drugs addition.
Contact time (CT): the duration of drug presence in the organ bath
until its removal by washing .
• CT= 30 sec, NT=60 sec, washing 3 times 30 sec for each total
cycle time = 3 min.
Apparatus for experiments with isolated
amphibian and mammalian tissue:
Physiograph and thermostatically controlled organ bath.
Drugs acting on the isolated tissue:
• Isolated rabbit intestine has normal myogenic contraction which is
under the control of the Autonomic nervous system (ANS).
• The effect of drugs that act on the intestine could be a
stimulatory i.e. increase in the intestinal contraction or
inhibitory i.e. decrease in the intestinal contraction.
• This increase or decrease in muscle contraction is
observed as follows:
↑ amplitude
↓ amplitude
• Or
↑ tone
↓ tone
• Or
↑ tone & amplitude
↓ tone & amplitude
The nervous control of intestine:
Intestine :
 Is composed of(duodenum,jejunum,ileum) .
 It is a Smooth muscle has myogenic contraction (i.e.
spontaneous rhythmic contraction).
N.B this rhythmicity due to its pendular movement.
 Is involuntary muscle undergoes the control of nervous
system(sympathetic and parasympathetic).
The intestine innervated by ANS :
Sympathetic
Transmitter: noradrenalin
mainly, Adrenaline .
Receptors present in
(intestine): Adrenergic: α1,B2
Action:
inhibitory (↓contraction)
relaxation .
Parasympathetic
Acetylcholine (Ach) .
Cholinergic:
Central: Nicotinic (Nn) .
and peripheral: muscarinic
(M3)
excitatory
(↑contraction)
contraction
Types of receptors present in the intestine:
 Adrenergic.
 Cholinergic.
 Histaminergic.
 Purinergic.
 Serotinergic.
Stimulant Drugs (Spasmogenic)
A- Autonomic cholinergic stimulants , act on:
1-Nicotinic Neuronal receptor:
e.g. Nicotine (small doses)= Diluted Nicotine
Mechanism of Action:
membrane Diluted Nicotine activates Nn receptors in the cell
This will activate Na+/K+ channel
 Na+ influx
increase intracellular Na+
 rapid depolarization
 intestinal smooth muscle contraction .
2)Muscarinic Receptors (M3 subtypes):
e.g. Cholinomimetic
1-Acetylcholine or its derivative : e.g. .Methacholine, Bethanecol,
Carbachol.
2- Naturally occurring alkaloid : e.g. Pilocarpine.
Mechanism of Action :
1. Acetylcholine binds with M3 receptors  activation of PLC
( phospholipase C) .
2. PLC hydrolyzes PIP2(phosphotidyl inositol biphosphate) into
DAG and IP3 .
DAG=diacylglycerol.
IP3= inositol triphosphate.
3. DAG activates Ca+ + channel  increase intracellular Ca+ +.
4.IP3 increase Ca+ + release from intracellular stores.
3,4  increase intracellular Ca+ +  intestinal smooth muscle
contraction.
Stimulant Drugs (Spasmogenic)
B-Direct smooth muscle stimulants :
e.g. Barium Chloride
Mechanism of Action:
BaCl2 stimulates the muscle fiber  increase muscle contraction.
Inhibitory drugs (Spasmolytics)
A-Autonomic adrenergic stimulants:
e.g. Adrenaline, Noradrenaline
Mechanism of Action:
Adrenaline will stimulate α1 and ß 2 receptors in the intestinal
smooth muscle
 inhibits tone and motility
 decrease intestinal contraction .
Inhibitory drugs (Spasmolytic)
B-Direct Inhibitory Drugs :
e.g. Mgcl2
Papaverine
Mechanism of Action :
Mgcl2 inhibits the contraction of muscle fiber  muscle
relaxation .